Very rare that WN would turn a plane in 15 min unless an extremely light load both inbound and outbound. Takes that long just to unload and load bags.

About the shortest scheduled turn I have seen is 25 min., and ofter 30 min. Still that is quite fast considering all that has to be done.

Then again, what is turn time? Is it time plane docks at gate until it starts backing off? Usually I see plane may be loaded and pax seated, but we do not start backing out for 3-8 min later.